Presently, many electronic instruments exist that use the ultrasound doppler method in detecting, for example, blood flow or fetal heartbeat. These instruments generally use one of two methods to remove from an input signal describing an area of investigation the usually large signal component indicative of stationary objects within the area of investigation, and to reveal the usually weak doppler signal component indicative of moving objects within the area of investigation.
One method involves early demodulation of the input signal after receipt and sharply cutting-off the lower components of the signal after demodulation. This method avoids the problem of saturation that could be caused by the usually large stationary signal component when the input signal is amplified before demodulation, but this method requires demodulator and top-stage baseband (audio frequency) amplifier with an excellent noise figure. Noise figure (NF) is defined as follows: ##EQU1## WHERE (S/N) represents signal-to-noise ratio.
Another method involves the use of a sharp notch filter to eliminate or remove the stationary signal component. But, such filters are expensive for they must not only suppress the carrier frequency and frequencies within about 100 Hz of the carrier frequency inherent in the input signal, but must also pass frequencies that are more than 100 Hz away from the carrier frequency.
Both of the above methods produce distortion or cross modulation and otherwise increase the overall noise figure of an instrument or system when removal of the stationary component and amplification of the weak doppler component are performed.